1. Field of the Invention
The present invention relates to an electrophotographic member for use in a fixing member of a heat fixing assembly in an electrophotographic image forming apparatus such as electrophotographic copier and electrophotographic printer, and a heat fixing assembly for use in an electrophotographic image forming apparatus.
2. Description of the Related Art
Products with reduced power consumption have recently been desired in the field of business machines. For a measure for reducing power consumption in an image forming apparatus such as electrophotographic copier and laser beam printer, the thermal capacity of a heat fixing unit has been made progressively reduced. Known examples of such an on-demand type heat fixing unit include the following (1) and (2).
(1) A heat fixing unit including a ceramic heater arranged inside a film-shaped rotating body and a pressure roller which co-operates with the ceramic heater through the film-shaped rotating body so as to form a heating nip part, where an image on a recording material is heated by heat from the ceramic heater in the nip part.
(2) An electromagnetic induction heating-type heat fixing unit in which a film-shaped rotating body or a fixing roller itself generates heat.
In these circumstances, further acceleration in the first print time and energy saving have been recently under way, requiring further shortening of the heating startup time and a reduction in power consumption of a fixing assembly, in particular. Consequently, the pressure roller for use in the heat fixing assembly is required to have “high thermal insulation properties”. The idea is that the reduction in thermal conductivity of the elastic layer of the pressure roller allows the less heat quantity to be removed from the heating body through the pressure roller during starting up of the heat fixing assembly, so that the temperature increase rate of the film-shaped rotating body in contact with the pressure roller or the fixing roller can be improved.
A method in which air voids are arranged in the elastic layer of a pressure roller in order to lower the thermal conductivity of the elastic layer is a general method. A method in which resin microballoons are utilized as one of the materials is known. It is important that the voids in the elastic layer are formed of connected open-cell foams and open to the outside so that the change in the outer diameter of the pressure roller can be restricted. In the case that the voids are not formed of connected open-cell foams or not open to the outside, gas inside the voids expands when heated, so that the outer diameter of the pressure roller is changed. The change in the outer diameter of the pressure roller occurs later than the increase in surface temperature of the pressure roller, so that the control of pressure roller-driven paper transportation becomes unstable even with use of a common device for detecting the temperature under pressure.
In order to form an elastic layer having voids of connected foams from aggregates of resin microballoons, it is required that the resin microballoons are aggregated in a liquid rubber composition (Japanese Patent No. 3969942).
Meanwhile, the incorporation of an electron conducting agent (carbon black) into a roller having an elastic layer including voids of resin microballoons has been proposed for the purpose of reducing the electrical resistance of the elastic layer (Japanese Patent No. 4003042).
The surface of a pressure roller electrostatically charged by friction with a fixing roller to make a pair with the pressure roller or by friction with paper may cause paper to wind around the pressure roller or may generate a so-called electrostatic off-set image, i.e. electrostatically scattered toner on a sheet of paper, in some cases. In order to prevent the phenomenon, an elastic layer which is made electrically conductive has been proposed (Japanese Patent Application Laid-Open No. H07-129008).
The present inventors attempted to incorporate a conducting agent such as carbon black into a liquid rubber composition which contains resin microballoons and an aggregating agent so as to make an electrically conductive elastic layer of a pressure roller, having connected foam voids formed from aggregate of resin microballoons. As a result, the aggregation of the resin microballoons was inhibited in some cases due to interaction between the conducting agent and the aggregating agent.
The effect of a conducting agent on the aggregation of resin microballoons is a new problem which has never been conventionally recognized. The present inventors have recognized that the problem should be definitely solved for obtaining a fixing member having an elastic layer with high thermal insulation properties and high electrical conductivity.